refactor(mail): migrate protonmail-bridge from pacman to nix

Move the ProtonMail Bridge off the AUR protonmail-bridge-core package
and onto nix/host.nix, consistent with the other migrated user-leaf
tools. Since the AUR package previously supplied the systemd user unit
(customized via a drop-in), ship a repo-owned
dot_config/systemd/user/protonmail-bridge.service instead: it runs the
nix binary by absolute %h/.nix-profile/bin path with --noninteractive
and folds the former drop-in's PASSWORD_STORE_DIR into the unit, so the
now-redundant protonmail-bridge.service.d/override.conf is removed.

Drop protonmail-bridge-core from meta/base.txt (the git send-email Perl
prereqs stay). No vm.nix change: the bridge is host-only and user units
are not symlinked on the headless VM.

5 files changed, 26 insertions, 4 deletions

diff --git a/dot_config/systemd/user/protonmail-bridge.service b/dot_config/systemd/user/protonmail-bridge.service
new file mode 100644
index 0000000..b664eba
--- /dev/null
+++ b/dot_config/systemd/user/protonmail-bridge.service
@@ -0,0 +1,14 @@
+[Unit]
+Description=ProtonMail Bridge (local IMAP/SMTP gateway)
+Documentation=https://proton.me/mail/bridge
+
+[Service]
+Type=simple
+# pass-backed credential vault (gpg-agent decrypts on first start).
+Environment=PASSWORD_STORE_DIR=%h/.local/share/password-store
+ExecStart=%h/.nix-profile/bin/protonmail-bridge --noninteractive
+Restart=on-failure
+RestartSec=5s
+
+[Install]
+WantedBy=default.target
diff --git a/dot_config/systemd/user/protonmail-bridge.service.d/override.conf b/dot_config/systemd/user/protonmail-bridge.service.d/override.conf
deleted file mode 100644
index 7e39182..0000000
--- a/dot_config/systemd/user/protonmail-bridge.service.d/override.conf
+++ /dev/null
@@ -1,2 +0,0 @@
-[Service]
-Environment=PASSWORD_STORE_DIR=%h/.local/share/password-store
diff --git a/meta/base.txt b/meta/base.txt
index 7b26ce9..756f776 100644
--- a/meta/base.txt
+++ b/meta/base.txt
@@ -153,7 +153,8 @@ xdg-user-dirs
 arkenfox-user.js
 
 # --- mail (host-side bits the org.mozilla.thunderbird flatpak depends on) ---
-protonmail-bridge-core
+# protonmail-bridge now comes from nix/host.nix (the bridge binary + its
+# repo-owned user unit dot_config/systemd/user/protonmail-bridge.service).
 # git send-email Perl prereqs (SMTP via local Bridge on 127.0.0.1:1025)
 perl-authen-sasl
 perl-mime-tools
diff --git a/nix/host.nix b/nix/host.nix
index ecaf464..0565f1b 100644
--- a/nix/host.nix
+++ b/nix/host.nix
@@ -62,6 +62,15 @@ in
     # flatpak sandbox.
     external-editor-revived
 
+    # ── Mail: ProtonMail Bridge ───────────────────────────────────────────────
+    # Headless IMAP/SMTP gateway (127.0.0.1:1143/1025) consumed by the
+    # Thunderbird flatpak and `git send-email`. No system coupling: it's a
+    # pure user daemon with a repo-owned user unit at
+    # dot_config/systemd/user/protonmail-bridge.service (run --noninteractive,
+    # pass-backed vault via PASSWORD_STORE_DIR). Replaces the AUR
+    # `protonmail-bridge-core`.
+    protonmail-bridge
+
     # ── Wayland session: bars, launchers, notifiers, daemons ──────────────────
     # Pure user-session GUIs/daemons — no system unit, no D-Bus activation
     # file under /usr/share/dbus-1, no login-manager session entry. The
diff --git a/systemd-units/user.txt b/systemd-units/user.txt
index 5002a6c..270e314 100644
--- a/systemd-units/user.txt
+++ b/systemd-units/user.txt
@@ -18,5 +18,5 @@ teams-xsight.service
 waybar.service
 wob.service
 
-# --- mail (overridden via drop-ins in dot_config/systemd/user/) ---
+# --- mail (repo-owned unit in dot_config/systemd/user/) ---
 protonmail-bridge.service