Redo file naming in the style of Lidarr and add "Rename now"

Naming: separate artist folder, album folder, track and multi-disc track
formats with presets, Lidarr-style tokens ({Artist Name}, {Artist
NameThe}, {Artist CleanName}, {Artist NameFirstCharacter}, {Album
Title}, {Album TitleThe}, {Album CleanTitle}, {Release Year},
{track:00}, {medium:00}, {Medium Format}, {Track Title}, {Track
CleanTitle}, {Track ArtistName}, {Genre}) with Lidarr's word-separator
and case variants, colon replacement modes and replace/remove for other
illegal characters. Empty brackets and separators left by missing values
are cleaned up. Defaults to Artist/Album/01 Title with Disc 01 folders
for multi-disc albums; earlier pattern/disc settings are converted.

Examples are now rendered by the server with the same code as real
renames, fixing the settings page showing paths that differed from what
would happen (e.g. disc 1 of a multi-disc album outside its Disc folder).

"Rename now..." checks one or both libraries, then lists every change
grouped by destination folder with per-file and per-folder selection,
skipped files with reasons and a Fix tags button, and warnings; applying
re-checks each file, renames, and rescans. The pre-action backup runs
first when enabled. Migration 9 stores the naming as JSON.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/engine.go

@@ -46,6 +46,9 @@ type Engine struct {
 	beforeAction func(ctx context.Context, reason string) error
 	tagCache     tagCache
 
+	plansMu sync.Mutex
+	plans   map[int64]*organizePlan // latest "rename now" preview per pair
+
 	jobsMu sync.Mutex
 	jobs   map[int64]*Job
 	nextID int64