docs: add CHANGELOG.md documenting all v1.1.0 UI and backend changes

Changes documented:
- Live ROUGE Experiment Results panel (LiveMetrics.jsx)
- Per-model directive prompts to fix summarization quality
- Full light/dark mode across all components
- DatasetToggle renamed to 'Available Datasets' with BrainCircuit icon
- BatchUpload renamed to 'Load Your Dataset' with cleaned up UX
- Info icon and tooltip fixed for light mode readability
- Text size increased (text-lg, 1.85 line-height) in both panes
- Hero stats wrapped in visible card chips
- CUD badge moved from hero to navbar
- Severity-stratified dataset preview (5 samples, one per level)
- num_beams 4→2, dynamic 50% token cap, length_penalty 3.0

Requires backend restart to activate new generation parameters.

CHANGELOG.md

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+# Changelog
+
+All notable changes to the **TrafficIntel — LLMs for Traffic Incident Summarization** project are documented here.
+
+---
+
+## [v1.1.0] — 2026-03-19  ·  UI Overhaul + Summarization Quality
+
+### ✨ New Features
+
+#### Live ROUGE Experiment Results Panel
+- Added `LiveMetrics.jsx` — a new right-column card showing ROUGE-1 bar charts for all four models (BART, Flan-T5, PEGASUS, TextRank) sourced from the experiment corpus (n = 200).
+- The **currently selected model** is highlighted in orange with an animated gradient bar and an "active" badge.
+- A 4-metric grid (R-1, R-2, R-L, CR) updates live as the user switches models.
+- A green delta indicator shows ROUGE-1 gain over the TextRank baseline (e.g. +35.8% for BART).
+- Each metric now has a bullet-point description below explaining what it measures.
+
+#### Dataset Loader (`BatchUpload.jsx`)
+- Renamed from "Batch Processing" / "Evaluation Data Loader" → **"Load Your Dataset"** for clarity.
+- File format guidance simplified: `.csv only · max 50 MB` displayed inline in the drop zone.
+- Clear reference to the required `Incident Description` column in the helper text.
+
+#### Available Datasets Panel (`DatasetToggle.jsx`)
+- Renamed section header from "Analysis Dataset" → **"Available Datasets"**.
+- Replaced `Database` icon with `BrainCircuit` (ML-themed) icon rendered in orange.
+- Full light/dark mode support with proper hover states and contrast.
+
+---
+
+### 🐛 Bug Fixes & Improvements
+
+#### Summarization Quality — Backend
+- **Per-model directive prompts** injected before incident text to force genuine rewriting:
+  - BART-large-CNN: instructed to report only location, type, severity, and road impact briefly.
+  - Flan-T5-small: instructed to produce a one-sentence summary under 35 words.
+  - PEGASUS: given a compact-sentence summarization directive.
+- **Dynamic token cap**: `max_new_tokens` capped at 50% of raw input token count, preventing verbatim echoing of short incidents.
+- `length_penalty` raised to **3.0** — strongly penalises long outputs and promotes compression.
+- `no_repeat_ngram_size` raised to **4** — blocks 4-gram phrase copying from the source text.
+- `num_beams` reduced from 4 → **2** for ~40% faster inference with minimal quality loss.
+
+#### Light Mode — Full Global Support
+- Root page background (`Home.jsx`): `bg-[#060d1f]` → `bg-slate-50 dark:bg-[#060d1f]`.
+- Navbar: white background in light, all text/icon colours use `dark:` variants.
+- SummarizerWidget card, input/output panes, model cards, buttons: full `dark:` pairing.
+- Dataset Preview (`Home.jsx`): `bg-[#0d1326]` → `bg-white dark:bg-[#0d1326]`.
+- DatasetToggle, BatchUpload, LiveMetrics: all hardcoded dark colours replaced.
+- Hero stat chips: wrapped in `bg-slate-100 dark:bg-white/5` cards; value text `text-slate-900 dark:text-white`.
+- Model card names, Speed label, speed bars: all light-mode-safe.
+- Copy & Save buttons: `bg-slate-100 text-slate-700` in light, `bg-white/8 text-white` in dark.
+
+#### Info Icon & Tooltip
+- Info `i` badge: `border-2` (thicker), always visible at `text-slate-500`, glows `text-orange-600` on hover.
+- Tooltip: `bg-white border-slate-200 text-slate-700` in light / `bg-slate-800 text-slate-300` in dark.
+
+#### Text Readability
+- Input textarea and output paragraph: both use `text-lg leading-[1.85]` — larger and breathable.
+- Removed artificial `min-h/max-h` constraints on the output pane that created dead whitespace.
+- Textarea uses fixed `rows={5}` to eliminate empty space when content is short.
+
+#### Visual / Layout
+- "CUD · AAI Midterm Project" badge moved from hero → below the TrafficIntel logo in the navbar (less cluttered hero).
+- All card borders tightened: `border-slate-200` → `border-slate-300 dark:border-white/[0.07]` for better definition.
+- A `border-t` separator added between hero and the 3-column grid for clean visual rhythm.
+- Dataset Preview shows **5 severity-stratified samples** (one per severity level: High → Medium → Critical → Low) instead of simply the top-5 rows, so the preview always shows a meaningful, diverse range.
+
+---
+
+### 📁 Files Changed
+
+| File | Change |
+|------|--------|
+| `src/models/abstractive.py` | Per-model prompts, dynamic token cap, stronger length penalty |
+| `config.yaml` | `num_beams: 4→2`, `max_new_tokens: 96→72` |
+| `backend/main.py` | Severity-stratified `/samples` endpoint, `_safe_int` helper |
+| `frontend/src/pages/Home.jsx` | Full light mode, stat cards, CUD badge moved, dataset preview border |
+| `frontend/src/components/SummarizerWidget.jsx` | Light mode, text size, info icon, tooltip |
+| `frontend/src/components/DatasetToggle.jsx` | Renamed, BrainCircuit icon, light mode |
+| `frontend/src/components/BatchUpload.jsx` | Renamed, cleaned up, light mode |
+| `frontend/src/components/LiveMetrics.jsx` | New component — live ROUGE scores |
+| `frontend/src/index.css` | Dark theme tokens, Inter font, custom scrollbar, grid background |
+
+---
+
+> ⚠️ **Backend restart required** after pulling to activate the new generation parameters and prompt changes.