feat(config): configurable window-title and tab-title format strings

[HermeticOrmus]

Why

Refs #720. cosmic-term already ingests OSC 7 (working directory) for the new-tab-cwd-inherit feature, but never surfaces it in the window or tab title. Users who want to see what directory the active shell is in have to run `pwd`. Ghostty's default title format includes the cwd; alacritty supports `window.title` formatting; cosmic-term should match.

What changed

`src/config.rs` gains two fields:

• `window_title_format: String` — default `"{title} — {app}"` (preserves current behaviour)
• `tab_title_format: String` — default `""` (empty = use the shell-set title verbatim, current behaviour)

Both accept the placeholders `{title}`, `{cwd}`, `{cwd_basename}`, `{app}`.

`update_title()` in `src/main.rs` calls a new `config::render_title_format` helper to substitute. Defaults reproduce the existing UX exactly — no behaviour change unless the user opts in.

Example user config that mirrors Ghostty's default:

```ron
window_title_format: "{cwd_basename} — {title}",
tab_title_format: "{cwd_basename}",
```

How to test

```bash
cargo build --bin cosmic-term
cargo test --bin cosmic-term config::tests::

6 tests: substitutes title+app, substitutes cwd, cwd_basename, missing cwd,

unknown placeholders left alone, substitution order edge case

```

All 6 pass locally on Linux x64.

Manual:

• Default config: window title shows `bash — COSMIC Terminal` (unchanged).
• Set `window_title_format: "{cwd_basename} — {title}"`: window title becomes `cosmic-term — bash` while cwd matches.
• Unknown placeholders are left literal.

Notes

• `cwd_basename` is substituted before `cwd` in the renderer because `{cwd_basename}` literally contains the substring `cwd}`. Test `cwd_basename_must_match_before_cwd_to_avoid_partial_replacement` guards this.
• Active CWD is read via the existing `Self::active_terminal_working_directory()`, which uses `/proc/<shell_pid>/cwd` — no new syscalls beyond what was already happening for the cwd-inherit path.
• Branch is based off commit 7ce2974 to match the fork's master baseline (HermeticOrmus/cosmic-term hasn't been rebased onto recent upstream master); the diff against pop-os/cosmic-term/master should still be minimal — just the two files in `src/`.

[jackpot51]

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