Write a contributing guide, and fix what the forge move left stale (#96)

(docs) Update all the docs to be up to date with current status quo

.forgejo/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Something doesn't work the way it should
+title: ""
+labels:
+  - bug
+---
+
+### What happened
+
+
+### What you expected
+
+
+### Steps to reproduce
+1.
+2.
+3.
+
+### Environment
+- Calendula version: <!-- Settings → bottom of the screen -->
+- Android version:
+- Device:
+- Installed from: <!-- official F-Droid / the self-hosted repo / built from source -->
+- Affected calendar: <!-- Google, CalDAV (DAVx5, Nextcloud, …), on-device/local,
+                         subscribed/WebCal, birthdays — provider behaviour differs
+                         a lot per account type, so this often points straight at
+                         the cause -->
+- Time zone: <!-- only if the problem involves dates or all-day events -->
+

.forgejo/ISSUE_TEMPLATE/config.yml

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+# Kept enabled so anything that doesn't fit the four templates still has a way
+# in (the `ToDo` label exists for exactly those).
+blank_issues_enabled: true
+
+contact_links:
+  - name: Translate Calendula
+    url: https://weblate.dev.jeanlucmakiola.de/engage/calendula/
+    about: >-
+      Translations are managed on Weblate, not here — it owns every values-*
+      file, so a hand-edited translation gets overwritten on the next sync.
+      No coding needed: pick or request a language and translate in the browser.
+
+  - name: Contributing guide
+    url: https://codeberg.org/jlmakiola/calendula/src/branch/main/CONTRIBUTING.md
+    about: >-
+      Before opening a pull request: the issue-first workflow, which release
+      branch to target, how to build (there's a submodule), and the
+      architectural rules a change is reviewed against.

.forgejo/ISSUE_TEMPLATE/crash_report.md

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+---
+name: Crash report
+about: Report a crash. Calendula can capture this for you (Settings → Report a problem, or the prompt after a crash) — it copies the report to your clipboard and prefills this form.
+title: "Crash: "
+labels:
+  - bug
+  - crash
+  - priority/high
+---
+
+<!--
+Thanks for reporting a crash in Calendula!
+
+If the app prefilled this for you, the crash report is already below — just add
+what you were doing and submit. Otherwise, paste the report from your clipboard
+into the code block. The report contains only app/Android/device versions and the
+stack trace — no personal data or calendar content.
+-->
+
+### What happened
+
+
+### Crash report
+
+```
+(paste the crash report here)
+```

.forgejo/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea or improvement
+title: ""
+labels:
+  - feat
+---
+
+### What would you like Calendula to do?
+
+
+### Why — what problem does it solve?
+
+
+### Anything else
+<!-- mockups, examples from other apps, alternatives you considered -->

.forgejo/ISSUE_TEMPLATE/question.md

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+---
+name: Question
+about: Ask how something works or get help using Calendula
+title: ""
+labels:
+  - question
+---
+
+### Your question
+
+
+### What you've tried
+<!-- so far, if anything -->
+
+
+### Context
+- Calendula version: <!-- Settings → bottom of the screen -->
+- Android version:
+- Device:

.forgejo/PULL_REQUEST_TEMPLATE.md

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+<!--
+Thanks for contributing to Calendula!
+
+Please skim CONTRIBUTING.md if you haven't:
+https://codeberg.org/jlmakiola/calendula/src/branch/main/CONTRIBUTING.md
+
+Two things it's easy to get wrong:
+  • Features need a discussed issue first — an undiscussed feature PR may be
+    closed unmerged even when the code is good.
+  • Target the release branch for your issue's milestone (milestone 2.18.0 →
+    release/v2.18.0), not main. If you targeted main, just say so below and it
+    will be retargeted.
+-->
+
+### What this changes
+
+
+### Why
+
+<!-- Closes #123 — link the issue this implements or fixes. -->
+
+
+### How it was tested
+
+<!--
+Which of these ran green, and anything you exercised by hand. On-device notes
+are especially useful for UI changes.
+
+  ./gradlew lint test assembleDebug
+  python3 scripts/check_translations.py
+-->
+
+
+### Checklist
+
+- [ ] There's an issue for this, and (for a feature) it got a go-ahead
+- [ ] Targeting the release branch for that issue's milestone — or `main`, noted above
+- [ ] `./gradlew lint test assembleDebug` passes locally
+- [ ] No `values-*/strings.xml` touched (Weblate owns those; new English strings in `values/` are fine)
+- [ ] `CHANGELOG.md` updated under `## [Unreleased]`, if the change is user-visible
+- [ ] No planning or design documents committed