v0.4.1 — thumbnails + grid view

Added

• WebP thumbnails + grid view UI (phase 4 user-visible tier).
  • perima-media::ThumbnailGenerator writes 256px WebP thumbnails
    (Lanczos3 resize, aspect-preserving) to
    <data_dir>/thumbnails/<aa>/<hash>.webp. Atomic write via
    .tmp + fs::rename — mid-write crashes cannot leave a
    half-written file.
  • V003 migration adds thumbnail_path + thumbnail_status to
    file_metadata (nullable, additive; existing v0.4.0 rows
    read as "not yet processed"). Partial index on
    thumbnail_status = 'pending' AND deleted_at IS NULL supports
    a future perima thumbnail retry command.
  • MetadataRepository::update_thumbnail — separate trait method
    so the queue worker's thumbnail result always persists without
    colliding with upsert_metadata's Unchanged equivalence proxy.
    Wrapped in BEGIN IMMEDIATE.
  • MetadataQueue worker now calls the thumbnailer after metadata
    extraction for image/video MIMEs; on success writes
    thumbnail_status = 'ready' + absolute path; on failure writes
    'failed' and continues (no worker abort).
  • ThumbnailGenerator::disabled() constructor + new
    perima scan --no-thumbnails flag shortcircuit the thumbnail
    write path for users wanting faster scans.
• Desktop grid view. New FileGrid component renders 200px
  tiles with thumbnails via Tauri's asset protocol
  (convertFileSrc). Header gains a Table / Grid toggle; default is
  Table for v0.3.x UX continuity. Placeholder icons for pending /
  failed / unknown tiles. 3 new vitest tests (14 total).
• tauri.conf.json enables assetProtocol with scope
  $APPDATA/perima/thumbnails/** + broad ** fallback.

Changed

• Desktop app now calls list_files_with_metadata (added in v0.4.0)
  for both Table and Grid views. FileWithMetadataPayload gains
  thumbnail_path + thumbnail_status fields.
• Tauri dep gains the protocol-asset feature flag.