fix(admin): rename 'Scan All Libraries' to 'Force Rescan All Libraries'

Owner: the dashboard button's label under-promised what it does. The
action is a FORCE rescan (scanner.SetForce(true) via the scan worker):
every file re-read and re-extracted with original-scan semantics, hash
identifiers recomputed — the admin's backfill tool, per the scanner's
own source comments. The shallow scans (per-library button, per-folder
scan, processing-issues retry) deliberately skip unchanged files and
keep their 'Scan' labels; the naming now carries that distinction.

Subtitle updated to state the real semantics ('Re-reads every file and
re-extracts metadata and hashes — slow but thorough') instead of the
old 'Re-scan existing files and detect new items', which described the
shallow path. ui-controls.md gains the canonical glossary entry (old
label recorded under also: so anything still quoting it resolves); no
doc quoted the old label yet, so no doc edits were needed.

docs/developer/ui-controls.md

@@ -212,6 +212,12 @@ label: Scan Interval (seconds)
 where: Settings page → Scanning section
 does: Sets the seconds between automatic scans.
 
+label: Force Rescan All Libraries
+where: /admin dashboard → Quick Actions
+does: Force-rescans every library — re-reads each file, re-extracts metadata with original-scan semantics, and recomputes hash identifiers. Unlike the shallow per-library and per-folder scans, nothing unchanged is skipped; this is the admin's backfill tool.
+also: Scan All Libraries
+note: renamed 2026-11-02 — the old label under-promised a heavy operation
+
 label: Resume rewind
 where: audio player → player settings; profile settings
 does: Rewinds playback a configured few seconds when you resume.