docs: add UI improvements design for sidebar + tabbed layout

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

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 # Linting
 .ruff_cache/
 
-# Docs
-docs/
+# Docs (ignore all except plans/)
+docs/*
+!docs/plans/
 
 # IDE / OS
 .DS_Store

docs/plans/2026-03-04-ui-improvements-design.md

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+# UI Improvements Design
+
+## Goal
+
+Improve the MADLAD-400 translation app UI to serve both as a research tool and a demo showcase, targeting the research community.
+
+## Layout
+
+Two-column design:
+
+- **Left sidebar (~25%)**: Target language dropdown + generation parameters (always visible)
+- **Main area (~75%)**: Tabbed interface with "Single" and "Batch" tabs
+- **Header**: Title, description, and paper link across the top
+
+## Sidebar — Generation Parameters
+
+Target language dropdown (moved from main area) plus three generation parameters:
+
+| Parameter | Control | Default | Range |
+|---|---|---|---|
+| Max new tokens | Slider | 512 | 1–1024 |
+| Num beams | Slider | 1 | 1–10 |
+| Temperature | Slider | 1.0 | 0.1–2.0 (step 0.1) |
+
+All parameters pass directly to `model.generate()`.
+
+## Single Translation Tab
+
+Same as current UI, relocated into tabbed layout:
+
+- Input textbox (English, 4 lines)
+- Output textbox (read-only, 4 lines, copy button)
+- Translate button
+- 4 examples at the bottom
+
+## Batch Translation Tab
+
+- File upload accepting `.txt` and `.csv`
+  - `.txt`: one sentence per line
+  - `.csv`: expects a `text` column (other columns preserved)
+- Downloadable output file
+  - `.txt` input -> `.txt` output (translated lines)
+  - `.csv` input -> `.csv` output with `translation` column appended
+- Gradio progress bar for batch advancement
+- No examples — format documented via placeholder text
+
+## Error Handling
+
+- **Empty input**: Disable Translate button when no input/file
+- **Large batches**: Rely on HF Spaces GPU timeout; return partial results with warning if cut short
+- **Invalid CSV**: Gradio error if `text` column missing
+- **Temperature + beam search**: Show `gr.Info()` notice when `num_beams > 1` and temperature is non-default (temperature has no effect with beam search)
+
+## Testing
+
+**Fast tests** (no CUDA):
+- UI structure: sidebar, tabs, all components render
+- Batch parsing: `.txt` and `.csv` parsing (valid and invalid)
+- Parameter validation: defaults and ranges
+
+**Slow tests** (CUDA + model):
+- Single translation with non-default generation params (e.g., `num_beams=4`)
+- Batch translation with small `.txt` and `.csv` files
+
+New tests added to existing `test_app.py` and `test_langmap.py`.