feat(flutter): make transcript opening feel instant

[wolfiesch]

What changed

• paint a small encrypted transcript tail immediately for previously opened sessions
• refresh it with the bounded transcript.page host command before attaching the live stream
• load earlier pages on demand while preserving the reader’s scroll position
• cap visible paged history at 4,096 entries and reject cross-session or malformed page data
• keep each session cache separate and never persist live or backward-page cursors
• fall back to the existing snapshot-and-stream flow on older hosts

Repair pass

• rebuilt the stacked branch on current main after the Flutter client merged
• retained the newer reconnect, gap-recovery, and transcript-anchor behavior from main
• replaced plaintext preferences with the existing encrypted platform storage
• prevented duplicate initial page requests and kept the cached-state notice visible above the transcript

Local verification

• Flutter analysis
• all 159 Flutter tests with 74.61% line coverage
• Flutter web build
• focused cache, controller, protocol, fixture, and UI tests

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