refactor(flatpak): route mpv and thunderbird via flatpak; drop system pkgs

Both org.mozilla.thunderbird and io.mpv.Mpv are already installed via
flatpak, but several places still launched the system binaries (because
they were in PATH). Worse, `mpv` was kept on the host *only* for the
streamlink-launches-mpv path, and `thunderbird` was being pulled in as
a hard dep of external-editor-revived even though it was never the
mailer actually used. Untangle both.

Thunderbird
-----------
* dot_config/sway/executable_tb-toggle.sh,
  dot_config/sway/executable_tb-autostart.sh:
    swap `thunderbird` → `flatpak run org.mozilla.thunderbird`. The
    `app_id` matcher in sway config already targets the flatpak id, so
    the scratchpad-stash and Super+t toggle keep working unchanged.
* etc/pacman.conf:
    add `AssumeInstalled = thunderbird=999.0-1`. external-editor-revived
    (AUR) hard-depends on `thunderbird`; this satisfies the dep without
    installing the package. Run `sudo pacman -Rns thunderbird` after
    deploy to remove the now-unneeded system binary.
* meta/base.txt: document the AssumeInstalled trick next to the
  external-editor-revived entry.

mpv
---
* dot_config/streamlink/config: `player=mpv` → `player=flatpak run
  io.mpv.Mpv`. The flatpak already pulls in our ~/.config/mpv via the
  read-only filesystem override (see
  run_onchange_after_deploy-flatpak-overrides.sh.tmpl), so behavior is
  unchanged.
* dot_local/bin/executable_linkhandler: same swap for inline video URLs.
* dot_local/bin/executable_mpv: deleted. The wrapper only existed to
  bwrap /usr/bin/mpv into _sandbox-net-parser; flatpak's own sandbox
  supersedes that.
* dot_local/bin/executable__sandbox-net-parser,
  dot_local/bin/executable_streamlink: comment refresh — mpv is no
  longer one of the tools this wraps, and the streamlink wrapper now
  forwards to the flatpak player rather than nested-bwrap caveats.
* meta/base.txt: drop `mpv` from the host package list and update the
  surrounding comment.

README.md: refresh the media row of the stack table to match.

On-host steps:

    chezmoi apply -v
    sudo pacman -Syu                          # picks up AssumeInstalled
    sudo pacman -Rns thunderbird mpv          # safe now
    flatpak install -y flathub org.mozilla.thunderbird io.mpv.Mpv
    swaymsg reload                            # pick up new tb scripts

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index 2ff70c3..1da3e7f 100644
--- a/README.md
+++ b/README.md
@@ -32,7 +32,7 @@ My Arch Linux configuration, managed with [chezmoi](https://www.chezmoi.io/).
 | Browser            | [LibreWolf](https://librewolf.net/) (Flathub `io.gitlab.librewolf-community` for bubblewrap host-isolation), hardened via `user-overrides.js` + `userChrome.css` (kept under `firefox/` by name for recognizability)                                                                          |
 | Mail               | [Thunderbird](https://www.thunderbird.net/) (Flathub `org.mozilla.thunderbird`) against [ProtonMail Bridge](https://proton.me/mail/bridge) + Radicale (CalDAV/CardDAV); non-private prefs tracked under `thunderbird/`                                                                        |
 | Secrets & identity | [GPG](https://gnupg.org/) (commit signing + SSH auth via gpg-agent), [pass](https://www.passwordstore.org/)                                                                                                                                                                                   |
-| Media & viewers    | [mpv](https://mpv.io/) (native for streamlink/IPC + Flathub `io.mpv.Mpv` as the sandboxed default for browser/mail handoffs), [zathura](https://pwmt.org/projects/zathura/) (Flathub `org.pwmt.zathura`), [yazi](https://yazi-rs.github.io/)                                                  |
+| Media & viewers    | [mpv](https://mpv.io/) (Flathub `io.mpv.Mpv`; streamlink launches it via `flatpak run`), [zathura](https://pwmt.org/projects/zathura/) (Flathub `org.pwmt.zathura`), [yazi](https://yazi-rs.github.io/)                                                                                       |
 | Code quality       | stylua + [selene](https://github.com/Kampfkarren/selene), [shfmt](https://github.com/mvdan/sh) + [shellcheck](https://www.shellcheck.net/), [ruff](https://github.com/astral-sh/ruff), [taplo](https://taplo.tamasfe.dev/), [prettier](https://prettier.io/) — all wired through `just check` |
 
 Keybinds are documented in [`KEYBINDS.md`](./KEYBINDS.md).