Cascading server search with always-visible progress (Thunderbird for Android)

[mail4mail]

 **Problem**
 Searching in Thunderbird for Android only searches messages already synced to the device.
 The optional server search is hidden behind a small toolbar icon, only searches the single
 folder you started from, and gives almost no feedback: with no local hits you see a blank
 screen, progress texts get overwritten while results arrive, and failures only flash as a
 short toast. Users regularly conclude that "search is broken" (see the blank-screen reports).

 **Proposal**
 1. When a manual search finds nothing locally, show a clear message with a tappable prompt
 to search on the server (instead of a blank screen).
 2. Let the server search cascade: current folder first, then the remaining folders of the
 account, then the other accounts - sequentially, skipping accounts whose backend cannot
 search server-side (e.g. POP3), and continuing past folders that fail.
 3. Show a persistent status bar with a spinner above the result list for the whole duration:
 "Searching <account> / <folder> on server (3 of 14)" and "Downloading message 2 of 8 from
 <folder>". End states stay visible in the list ("No messages found on the server",
 "Remote search failed").

 **Working prototype**
 I have a working implementation on a fork (branch `feature/cascading-server-search`):
 https://github.com/JustNotesa/thunderbird-android/tree/feature/cascading-server-search
 including screenshots and a test APK in the releases. Happy to adapt it to maintainer
 feedback and to the new message list implementation, and to submit it as a PR once the
 feature is accepted. The implementation was built with AI assistance (Claude).

[Jon]

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