feat(ui): sort columns + per-panel filter

[VirusAlex]

Summary

Two pieces of file-manager UX both panels were missing.

Clickable sort columns

• Click Name / Size / Modified in the column header → sort by that column ascending.
• Click the same column again → flip to descending.
• A small ▲ / ▼ marks the active column in accent colour.
• The "dirs first → files → symlinks" grouping is preserved (file-manager convention) — the chosen column orders within each group. So "Sort by Size desc" gives you "all dirs alphabetically, then files biggest-first".
• Sort state persists across breadcrumb navigation within a panel session.

Per-panel filter

A new row between the breadcrumb and the filelist with a text input.

• Substring match (case-insensitive) on entry name.
• 150 ms debounce so typing doesn't thrash.
• Esc clears, × button clears.
• Filter does NOT recurse — it operates on the current view only. Finding files deeper still needs the cross-tree comparison via /api/browse/stats, which is a separate feature.
• Filter resets on directory navigation (stale filter from another folder is rarely what you want).

Selection semantics under filter

Action	Behaviour
"Select all" (header checkbox)	Toggles ONLY the visible (filtered) entries. Selecting hidden ones silently would surprise.
Selections made BEFORE filter	Survive while filter is active. Toggle filter off → they reappear.
allSelected()	True iff every visible entry is in the selection.

Implementation

Pure client-side: ~50 lines in browser.js (state, displayEntries(), _compareEntries, setSort, clearFilter), header markup tweaks in index.html for both panels, and .col-sortable / .col-sorted / .panel-filter styles in style.css. No API changes, no Java touched.

Test plan

• Local mvn compile clean; mvn test -Dtest=ArchitectureTest 8/8 pass.
• CI green.
• Manual: navigate into a folder with mixed dirs+files+symlinks; verify dirs always come first; toggle each column header; verify the arrow glyph follows.
• Manual: type into the filter field; verify rows filter live, ↑↓ in keyboard tests, Esc clears.
• Manual: select some entries, then filter; check Header → "Select all" toggles only the visible ones; clear filter; verify pre-filter selections still there.
• Manual: navigate to a different folder; verify filter resets (sort prefs persist).

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