sync(batch-3): backport mobile stack-nav polish and upload guardrails

[tyreseluo]

Summary

This batch continues incremental upstream alignment while preserving existing robrix2 customizations.

1) Mobile Stack Navigation polish

• Backported dynamic mobile stack-navigation follow-up adjustments on top of existing robrix2 behavior.
• Added safe-area and header/body spacing refinements for mobile stack views.
• Added script-reapply handling in SpacesBarWrapper to avoid hidden-state flicker after reapply.
• Kept current robrix2 page structure and settings integration intact.

2) Upload guardrails and UX improvements

• Added homeserver max-upload-size precheck before sending attachment.
• If file exceeds server limit, upload is rejected locally with a clear error and no send attempt.
• Introduced retryability semantics for upload errors:
  • non-retryable for deterministic errors (e.g. server size limit exceeded)
  • retryable for transient send failures
• Updated upload progress UI behavior:
  • hide Retry for non-retryable errors
  • improved long-text wrapping/readability in status and filename labels
• File picker path updated to async flow in RoomInputBar to avoid blocking UI thread during selection.

Files

• src/home/home_screen.rs
• src/home/invite_screen.rs
• src/home/room_screen.rs
• src/home/space_lobby.rs
• src/home/upload_progress.rs
• src/room/room_input_bar.rs
• src/shared/file_upload_modal.rs
• src/sliding_sync.rs

Validation

• cargo build passed locally after migration changes.

Notes

• This PR is intentionally scoped as one incremental migration batch for easier regression testing and rollback if needed.