Allow custom folder for downloaded attachments

[birdbox219]

Add an option in Settings under Files to select a custom directory for downloaded attachments.

• Add custom_media_dir setting serialized to settings.json.
• Support custom attachment path resolution in AppDirs::media_dir, falling back to the default cache directory.
• Update backend worker to direct downloads and outgoing media to the configured media directory.
• Add Change, Reset, and Open folder buttons in Settings UI.
• Relocate matching media files in archive.db when the folder changes.
• Preserve custom user folders on logout by only clearing media_cache_dir.

Testing:

• Unit tests for settings defaults and round trip (settings::tests::custom_media_dir_defaults_to_none, settings::tests::round_trips_through_disk).

• Unit test for path fallback and custom override (paths::tests::media_dir_uses_custom_path_when_configured).

• Unit test for live directory update and in-memory message path repair (app::tests::media_dir_changed_event_updates_settings_and_dirs).

• Manual verification with --demo --demo-page settings testing folder selection, path display, open folder,and reset.

  Checks run on Windows: fmt, clippy (default and all features), tests (default and all features), and cargo doc, all clean. Other platforms compile through the same code paths with no new platform-specific code.

[Copilot]

🟡 Changes recommended

Unresolved findings affect attachment relocation, cache cleanup, UI blocking, and in-memory path consistency.

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Pull request overview

Adds persistent custom attachment storage with folder selection, reset, and live path updates.

Changes:

• Persists and resolves a configurable media directory.
• Routes downloads and outgoing media through it.
• Adds Settings controls and backend path updates.

File summaries

File	Description
src/ui/settings.rs	Adds attachment folder controls.
src/settings.rs	Persists the custom directory.
src/paths.rs	Resolves the effective media directory.
src/model.rs	Adds media directory actions.
src/main.rs	Initializes the configured directory.
src/backend/worker.rs	Handles relocation, cleanup, and media routing.
src/backend.rs	Adds media directory commands and events.
src/app.rs	Applies changes and synchronizes loaded media paths.

Review details

Suppressed comments (4)

src/app.rs:1226

• This handler performs up to two blocking metadata checks for every loaded message on the UI thread. A custom directory may be slow, and this duplicates the worker's relocate_media scan. Have the worker return the repointed message paths and let the UI apply those results instead of probing the filesystem here.

                    for conversation in self.conversations.values_mut() {
                        for message in &mut conversation.messages {
                            let Some(media) = message.content.media_mut() else {
                                continue;
                            };

src/app.rs:1237

• relocate_media clears this row from the archive when neither the old path nor a file with the same name in the new directory exists, but this branch leaves the loaded message's media.path unchanged. An open conversation can therefore keep rendering the attachment as downloaded and try to open a nonexistent file until the chat is reloaded. Clear the in-memory path when no candidate is found so it stays consistent with the archive.

                            if !path.exists() {
                                let candidate = dir.join(name);
                                if candidate.exists() {
                                    media.path = Some(candidate);
                                }

src/app.rs:2428

• The advertised --demo --demo-page settings verification cannot exercise these actions: demo startup uses App::headless, whose detached backend drops commands, and no demo responder handles PickMediaDir or ResetMediaDir. The Change and Reset flows therefore do nothing in that mode; verify them with a real backend or add a demo response path.

            Action::PickMediaDir => self.backend.send(Command::PickMediaDir),
            Action::ResetMediaDir => self.backend.send(Command::ResetMediaDir),

src/backend/worker.rs:2589

• Changing the setting does not actually relocate existing attachments. relocate_media skips every row whose recorded old path still exists (src/backend/worker.rs:526-527), so selecting a new folder leaves the file and archive row in the old directory; Reset has the symmetric problem. Move existing files into the new directory and update their archive paths as part of the change so the configured folder applies to existing downloads.

                self.relocate_media();

• Files reviewed: 8/8 changed files
• Comments generated: 4
• Review effort level: Lite

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[Copilot]

Copilot review overview

🟡 Changes recommended

Media relocation, nested-directory cleanup, and opening an uncreated media directory remain unresolved.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1 · 1

Open (2)

• Changing media directory leaves existing attachments in the old cache · New
• Open media folder fails when the media directory does not exist · New

Resolved since last review (4)

• This accepts a folder inside the cache media tree, but logout still recursively removes…
• media can now be an arbitrary user-selected path, but this click handler still calls synchronous…
• This introduces a user-visible storage exception, but README.md still says attachments always live…
• The added test only checks dirs and settings; it never creates a loaded media message or…