feat(tui): list custom themes in picker

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Summary

Addresses the approved #5901 slice.

The /theme picker now:

• discovers valid user-authored overlays from $CODEWHALE_HOME/themes/;
• appends custom overlay rows after compiled themes in stable order;
• preserves the normalized raw custom:<name> selector in App state;
• highlights the active custom overlay and previews its resolved colors;
• preserves a custom selector when the picker is opened and committed without navigation;
• continues to persist intentional built-in-theme selection normally;
• ignores invalid, unreadable, oversized, and symlinked overlay files without breaking the built-in picker.

The configuration guide and synchronized changelogs document the picker behavior.

Tests

Passed locally:

• cargo fmt --all -- --check
• git diff --check
• ./scripts/sync-changelog.sh --check
• user-theme discovery tests: 4 passed
• theme-picker tests: 20 passed with RUST_MIN_STACK=16777216
• custom live App-state test: passed
• config-command tests: 94 passed
• cargo clippy -p codewhale-tui --all-targets --locked -- -D warnings

No provider credentials or network access are required.

No-Issue: This PR addresses the approved focused #5901 picker slice without automatically closing the broader issue.

[Hmbown]

Verified the clobber on main — apply.rs passed the base theme name as the picker's original, and app/init.rs dropped the normalized selector, so storing app.theme_name (and cloning it at apply.rs:1981) is the right fix; the custom:midnight preview/commit test and the live App-state test in config.rs are exactly the coverage this needed. Two asks: (1) are the new custom rows exercised at narrow width (the picker's list-when-narrow path at theme_picker.rs:202 has infra; a one-line assertion would pin it), and (2) confirm custom row labels route through the same localized label helpers as built-ins. Thanks for skipping invalid/oversized/symlinked overlays and for leaving prompts/text.rs untouched. Housekeeping: commit is missing a Signed-off-by trailer — git commit --amend -s when convenient.