Add colored last-seen dots to dialogs

[ObnubiladO]

Summary

Adds optional colored last-seen dots to dialog avatars, matching Forkgram Android's thresholds:

• Recent (under 15 minutes): yellow
• Moderate (under 30 minutes): orange
• Stale (under 60 minutes): red
• Online/future status: existing green dot

The setting is enabled by default and can be toggled in Forkgram Settings. Hidden or approximate statuses, bots, service accounts, and timestamps at least 60 minutes old do not receive an offline-colored dot.

Dependency

Draft until forkgram/lib_ui#1 is merged.

That PR adds the three palette tokens referenced by this branch. The Desktop submodule currently points to its rebased commit 31fde03 on top of the latest official lib_ui base.

Implementation notes

• Uses one shared session timer for all watched users.
• Recomputes deadlines when an exact timestamp changes within the same color bucket.
• Invalidates dialog rows when thresholds or the setting change.
• Preserves the outgoing badge color during fade-out, avoiding a brief green flash on colored-to-none transitions.
• Includes a focused classifier and scheduling regression test.

Verification

• Rebased onto Forkgram Desktop dev at v7.2.5 (45f04355f60b8c9489412bddfcff1677407c359b).
• Focused Debug last-seen test passed.
• Incremental Windows x64 Release build passed.
• Release executable passed an isolated runtime smoke test.
• Windows x64 installer packaging passed.
• The feature was manually tested successfully before this rebase.

Existing base-test mismatch

The 7.2.5 base's test_update_verify executable reports 57/58 because Forkgram's new embedded manifest intentionally defines stable and beta, while the inherited test still asserts that four Telegram channels exist. This feature branch does not modify the manifest, update verifier, or that test.