feat(ux): unify icons, fix editor filter UX, add back buttons v1.086

- Replace 9 OEM-dependent @android:drawable icons with bundled vectors
  (new: ic_rotate, ic_redetect, ic_crop_full, ic_back; reuse existing set)
- Editor: highlight the selected filter (green, same rule as camera menu);
  brightness slider now applies on top of the selected filter instead of
  silently discarding it (filterBase); slider resets on filter switch
- Editor: retake keeps ID mode/submode/front URI instead of dropping back
  to the generic camera
- Camera: flash/resolution/filter/settings dropdowns are now exclusive
  (no more overlapping panels); fix wrong contentDescriptions (close
  button was labeled "off"); larger HD touch target
- Headers: back button on group/result/OCR screens
- Main: empty state with icon + guidance text; "select all" label toggles
  to "deselect all" when everything is selected
- Extract hardcoded prev/next labels to string resources

Constraint: ViewBinding ids (title, pageInfo, empty, backButton) keep their current usage sites; filterBase always derives from flattened.
Confidence: high
Scope-risk: module
Reversibility: trivial
Directive: CameraActivity (507 lines) still exceeds the 300-line hard limit - split by concern (flash/resolution/filter/multi) in a follow-up.
Tested: assembleDebug + testDebugUnitTest exit=0; reviewer agent pass (0 blocker, 0 major).
Not-tested: on-device visuals (icon rendering, back buttons, filter highlight) - verify with the v1.086 sideload.
Related: HANDOFF.md release-polish item

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