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/developer/android-app.md

@@ -1,6 +1,6 @@
 # Android App — Design & Roadmap
 
-This document describes the planned native Android client for Bookhoard: a thin, offline-first reading app that treats the Bookhoard server as its backend. The relationship is the same as the audiobookshelf app to an audiobookshelf server, or the Kindle app to Kindle cloud — the server owns the library, sync, and conflict resolution; the app is a dedicated, mobile-first reading frontend with its own UI, designed independently of the web interface.
+This document describes the planned native Android client for Bookhoard: a thin, offline-first reading app that treats the Bookhoard server as its backend. The relationship is the same as any dedicated reader app to its server: the server owns the library, sync, and conflict resolution; the app is a dedicated, mobile-first reading frontend with its own UI, designed independently of the web interface.
 
 **Status**: Planning / pre-development
 **Companion repo**: `bookhoard-app` (separate repository, AGPL-3.0)
@@ -32,7 +32,7 @@ This document describes the planned native Android client for Bookhoard: a thin,
 
 ### Why native Android
 
-- The quality bar is the Kindle app. Page-turn latency, text layout fidelity, PDF rendering (`PdfRenderer`), and comic/manga image pipelines are platform-level strengths — and they are the *hard* parts in a WebView, not the easy parts.
+- The quality bar is a first-class native reader app. Page-turn latency, text layout fidelity, PDF rendering (`PdfRenderer`), and comic/manga image pipelines are platform-level strengths — and they are the *hard* parts in a WebView, not the easy parts.
 - Android-first removes the "share one codebase across two platforms simultaneously" constraint that motivates hybrid stacks.
 - Solo, AI-assisted development compresses the cost of native (code volume), while native's failure modes (well-documented platform APIs) are far easier to debug — alone or with AI — than cross-framework bridge/plugin bugs.
 - The target audience is the self-hosted community, best reached via GitHub Releases and F-Droid rather than app-store optimization.