feat(core,gui): gettext translations and language preference

The app, the tray and the search prompt route every user-visible string
through GNU gettext in the `pdfs` text domain, with named placeholders and
plural forms. po/ holds the template, update and build scripts, and catalogs
for de, en_GB, es, fr, it, nl, pl and pt_BR. Preferences gains a Language row
that overrides the system language from the next start (config `language`).

Packages compile the catalogs into /usr/share/locale and the desktop files
carry translated descriptions and keywords. docs/TRANSLATING.md describes
the workflow.

Cargo.lock

@@ -467,6 +467,12 @@ dependencies = [
  "digest 0.10.7",
 ]
 
+[[package]]
+name = "block"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d8c1fef690941d3e7788d328517591fecc684c084084702d6ff1641e993699a"
+
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -1814,6 +1820,26 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "gettext-rs"
+version = "0.7.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d5857dc1b7f0fee86961de833f434e29494d72af102ce5355738c0664222bdf"
+dependencies = [
+ "gettext-sys",
+ "locale_config",
+]
+
+[[package]]
+name = "gettext-sys"
+version = "0.26.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ea859ab0dd7e70ff823032b3e077d03d39c965d68c6c10775add60e999d8ee9"
+dependencies = [
+ "cc",
+ "temp-dir",
+]
+
 [[package]]
 name = "ghash"
 version = "0.5.1"
@@ -2829,6 +2855,19 @@ version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "47d9d19d1d6efa0109d2f65ff4c85cddd50bd572e5a00127ab10987290bcefae"
 
+[[package]]
+name = "locale_config"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08d2c35b16f4483f6c26f0e4e9550717a2f6575bcd6f12a53ff0c490a94a6934"
+dependencies = [
+ "lazy_static",
+ "objc",
+ "objc-foundation",
+ "regex",
+ "winapi",
+]
+
 [[package]]
 name = "lock_api"
 version = "0.4.14"
@@ -2868,6 +2907,15 @@ version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4050469837a6ff301cd14c1f8f24f88549e6d548f24f64e2148eb0f72cebc51f"
 
+[[package]]
+name = "malloc_buf"
+version = "0.0.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62bb907fe88d54d8d9ce32a3cceab4218ed2f6b7d35617cafe9adf84e43919cb"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "matchers"
 version = "0.2.0"
@@ -3189,6 +3237,35 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "objc"
+version = "0.2.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "915b1b472bc21c53464d6c8461c9d3af805ba1ef837e1cac254428f4a77177b1"
+dependencies = [
+ "malloc_buf",
+]
+
+[[package]]
+name = "objc-foundation"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1add1b659e36c9607c7aab864a76c7a4c2760cd0cd2e120f3fb8b952c7e22bf9"
+dependencies = [
+ "block",
+ "objc",
+ "objc_id",
+]
+
+[[package]]
+name = "objc_id"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c92d4ddb4bd7b50d730c215ff871754d0da6b2178849f8a2a2ab69712d0c073b"
+dependencies = [
+ "objc",
+]
+
 [[package]]
 name = "ocb3"
 version = "0.1.0"
@@ -3435,6 +3512,7 @@ version = "2.0.0"
 dependencies = [
  "anyhow",
  "async-channel",
+ "gettext-rs",
  "gtk4",
  "kamadak-exif",
  "ksni",
@@ -4055,6 +4133,18 @@ dependencies = [
  "syn 3.0.5",
 ]
 
+[[package]]
+name = "regex"
+version = "1.13.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f020237b6c8eed93db2e2cb53c00c60a8e1bc73da7d073199a1180401450218d"
+dependencies = [
+ "aho-corasick",
+ "memchr",
+ "regex-automata",
+ "regex-syntax",
+]
+
 [[package]]
 name = "regex-automata"
 version = "0.4.18"
@@ -4796,6 +4886,12 @@ version = "0.13.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "adb6935a6f5c20170eeceb1a3835a49e12e19d792f6dd344ccc76a985ca5a6ca"
 
+[[package]]
+name = "temp-dir"
+version = "0.1.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "83176759e9416cf81ee66cb6508dbfe9c96f20b8b56265a39917551c23c70964"
+
 [[package]]
 name = "tempfile"
 version = "3.27.0"