i18n: Comprehensive i18n fixes and workflow improvements, update Polish translation

[lukaszszczesniak]

This PR focuses on fixing broken i18n across the project, updating outdated translation templates, and improving the translation workflow for the future.
By the way, an update of Polish translations was also added to the pr.

Key Changes

1. Minor fixes in the source code (69f31a8)

• Fixed a possible typo in a label within welcome_page.blp.
• Added missing translatable marking in recent_picker.blp.

2. Translation workflow and automation (4f12d49, 1f0587f, 476a4dd)

• New Update Script: Added scripts/update-pot.sh to automate the extraction of translatable strings and updating the POT file. This script is analogous to the existing scripts/update-po.sh.
• Template Updates: Refreshed the main POT file and propagated changes to all existing PO files.

A key finding was that the previous POT file was missing many changes related to keyboard accelerators (the _ character in msgids). Because the msgids in the source code didn't match the translations, many existing translations simply weren't being applied. This PR synchronizes everything so that translations can work correctly again. Small change, big impact! Unfortunately, translators will still need to check translations and at least remove #, fuzzy marks from messages to make changes visible to the end user.

3. Updated TRANSLATING.md (9b951f6)

• Added instructions on how to update existing translations, in addition to creating only new ones, making it easier for future contributors to help with Gradia localization.
• Added information regarding previously mentioned keyboard accelerators.
• Fixed an incorrect path to the be.alexandervanhee.gradia.Devel.json file.

4. Updated PL translation (fc7bd95)

• Completely updated the Polish translation (pl.po) to match the new template, fixed broken strings and drastically increased the overall translation quality.

Quality Assurance

• Manual Work Only: All code changes and translations were done manually. No LLMs were used for generating code or translation strings to ensure accuracy and maintain the project's standards. Claude was used solely to assist with the description of this PR.
• Thoroughly Tested: I have tested the application extensively after these changes to ensure that the UI renders correctly, accelerators work as intended, and the translation workflow is robust.
• Organized Commits: The history has been carefully curated and described to make the review process as smooth as possible. 😇

Note to Maintainer

Feel free to push any additional changes or tweaks directly to this branch if needed. I'm happy to collaborate on any further adjustments to get these i18n fixes merged.