docs: remove speculative promise language, de-brand Android design analogy

Owner rulings from the admin-docs planning session:

- sync-guide: dropped 'Kindle and reMarkable are under consideration' —
  no such plans exist, and promising support for proprietary device
  interfaces we will never build sets false expectations. Device posture
  stays as the table states it: web, Android app, KOReader anywhere
  (including jailbroken Kindle hardware), native Kobo the sole exception.
- calibre-integration: cut 'Custom Calibre Columns (Future)' + 'Stay
  tuned!' — unshipped-feature promises do not belong in docs; calibre
  integration is a by-product of the scanner's sidecar support, not a
  roadmap item. The features list describing Calibre itself stays (that
  is Calibre's promise, not ours).
- android-app: de-branded the two Kindle references per owner's legal
  caution — 'the Kindle app to Kindle cloud' analogy and 'quality bar is
  the Kindle app' are now 'any dedicated reader app to its server' / 'a
  first-class native reader app'; engineering substance unchanged.

Governance rule going forward (codified in writing-ui-docs.md in a
follow-up): no future-improvement or coming-soon language anywhere
without explicit owner approval; anything found gets reported, not
silently kept or silently removed.

docs/user/calibre-integration.md

@@ -248,16 +248,6 @@ As long as a `metadata.opf` file exists in the folder, Bookhoard will import the
 
 ## Advanced Usage
 
-### Custom Calibre Columns (Future)
-
-Bookhoard may support Calibre custom columns in a future release. This would allow you to import:
-- Read status
-- Favorite flags
-- Custom metadata fields
-- User-defined categories
-
-Stay tuned for updates!
-
 ### OPDS Integration
 
 You can access your Bookhoard library (including Calibre-imported books) via OPDS from OPDS-capable clients: