Main window: fixes from the UI finish-gate review

[tneohcl]

A pre-release review of the main window (the ui-finish-gate-reviewer agent, rendering it offscreen in Dark and Light at default and minimum size) returned HOLD on six findings. This fixes all six.

1. Long filenames no longer break the settings column. Settings for "…" widened the fixed 456 px column and silently clipped the segmented controls (Processing collapsed to one huge "Automatic"). The scope label (_ElidedLabel) now elides in the middle and asks for no width; the full name is in the tooltip.
2. Convert acts only on unfinished videos. After a run it still said "Convert 4 Videos" and re-queued every row, re-encoding finished videos into name (1).mp4 copies. It now counts and queues only rows without _completed_output_path, so failed rows are retried; finished rows keep their result. With everything done it's disabled: "All videos converted — add more videos". The deferred start (Convert while analysis is running) uses the same check. The help already promised this behaviour.
3. Queue columns fit their content. Size was 60 px ("11.2GB" → "11.2…") and Status lost its % at 960 px. Video now stretches and elides; Duration, Size and Status are sized from the font to their widest real value (DropTreeWidget.fit_columns) and refit on font or style changes. Trade-off: Video can no longer be dragged wider.
4. Controls match the design decisions. No bold on selected segments or tabs (colour-only selection; the bold-width reservation is removed with it). Convert, Cancel and Open Folder are 28 px like the other toolbar buttons (Convert gets a 1 px border in its fill colour instead of border: none). Controls use the 4 px radius token instead of 6 px.
5. The focus ring no longer sits on the label. Qt drew the QSS outline inside the button, around the text. Buttons now show keyboard focus as their own 2 px border, with padding compensated so nothing moves; the queue draws one 2 px ring around the whole current row instead of a 1 px box around a cell. Clicks still show no ring.
6. Failures read without hovering. The worker now reports ffmpeg's last real error line instead of just "ffmpeg exited 1" (summarize_ffmpeg_failure). The row's second line shows it in red ("Failed: …"), and the finished outcome is a status headline: icon + sentence at section-title weight ("⚠ Completed with Issues").

Help (video-wont-convert, conversion-status) and the README are updated to match.

Also fixed: TestLeftPanelScrolling.test_no_scrolling_needed_at_default_size_with_expert_collapsed read the real config, so it failed whenever an earlier test had saved Expert as expanded. That showed up here because the new tests shifted the batch boundaries. It now uses _empty_qsettings(), like its sibling.

Tests: 28 new tests (6 classes in test_main.py, plus TestFailureReason in test_worker.py). 24 of them failed before the fixes and pass after; the other 4 are guards. TestSegmentedButtonBoldWidth is replaced by a plain fits-its-text check. All 32 batches of run_tests_chunked.py pass (621 tests).

Verified visually by re-rendering offscreen: long-name selection at 1186×720 and 960×640, the finished state with one failure, converting at minimum size, minimum size with 115% text, and zoomed Tab focus on Convert, a checked segment and a queue row. Not verified: the real desktop at 150% with the system accent and Breeze. In particular, with the default test accent the Convert focus ring (TEXT_ON_ACCENT) contrasts with the fill but not with the dark window behind it.

Not in this PR (the review's optional items): dropping Clear Queue's confirmation, the "Converting…" disabled primary slot, the "Automatic" button reading as a fifth segment, the repeated "Quality" label, the Expert box's collapsed padding, the "Preparing --" double hyphen, sentence case, menu mnemonics, the hard-coded 10pt body, and the two "open folder" buttons.

🤖 Generated with Claude Code

https://claude.ai/code/session_01E1Jny9Kg3YZX8GMCEStTvL