FIX: rest of the #17 backlog

[CodingDiego]

Closes #17.

These are the items #72 left open, plus the minor findings from its review. Each change was checked against the official v0.1.4 build by driving the real exe on a scratch profile (Opus verifier, messages only).

Item	v0.1.4	This PR
Clipboard change, 20,000 rows selected	63.6 ms of UI-thread CPU per change	1.6 ms (only Paste is re-evaluated)
Unpin from the sidebar	the window stalls up to 303 ms	under 3 ms (worker thread, one verb at a time)
Typing one character in the search box	starts a full recursive search	no search; 2 characters or Enter search
Folder written every ~20 ms with shell notifications	can show nothing until the writes stop	reloads about once a second
First session with icon_theme=default	types listed during the export keep the generic icon until restart	they get their own icon when the export lands; folder, search and archive tabs, no reload
Refresh in a case-sensitive folder with a.txt and A.txt	the selection moves to A.txt	the selection stays on a.txt (a case-only rename still reselects)
After a sort	the Shift+click anchor stays on the old row, and the focused row can be off screen	the anchor follows the focused item, which is scrolled into view
Sort with all 20,000 rows selected	227 ms	20 ms
Volume index (elevated search)	built while holding the cache lock, so every search waits	built per drive outside the lock; same-drive searches share one build

Not changed: if the first window is frozen while idle (a suspended process), a second launch waits 13 s (see the comment on #17). Windows does not report such a window as hung, so it cannot be told apart from a busy one.

Tests:

• cargo build/test --release --locked with -D warnings: app 83, core 87, all pass.
• New unit tests: forgetting_misses_finds_a_png_written_since, and selection_matches_exact_first_then_ascii_case_insensitively.
• Regression smoke subset (13 checks): all pass except sidebar-shell, which fails the same way on v0.1.4.

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