Add swipe to delete, swipe to restore, and interactive onboarding tutorial

[prince-0408]

Closes #622

Summary

Implements swipe-based text editing and a one-time interactive onboarding tutorial
for Scribe Keyboard. Users can now swipe left to delete and swipe right to restore
text, with haptic feedback on each step. First-time users are guided through both
gestures via a live practice overlay before it dismisses itself.

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What's changed

Swipe left — delete

• Swipe left anywhere on the keyboard to delete the previous word or character
• Deletion mode (word vs. character) follows the user's existing backspace preference
• Haptic tick fires on each 45px step of travel
• Gesture registers after 80px horizontal threshold; aborts default key click

Swipe right — restore

• Swipe right to restore the most recently deleted block from a stack, in sequence
• Stack is cleared automatically on cursor movement or normal key input to prevent
  stale restoration context

Interactive onboarding tutorial

• Shown once on first launch, controlled by swipe_tutorial_shown preference
• Overlay is non-blocking — touches pass through so users practice on the real keyboard
• "Scribe " is auto-inserted at tutorial start for immediate hands-on practice
• Skip dismisses at any step; Got it! dismisses from the success screen
• Both actions save the preference so the tutorial never reappears

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Tutorial steps

Step	Instruction	Advances when
1	Swipe left to delete "Scribe "	Word successfully deleted
2	Swipe right to restore "Scribe "	Word successfully restored
✅	Tap Got it! to finish	Preference saved, overlay closed

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Files changed

Logic

File	Changes
KeyboardView.kt	Added onSwipeLeft() and onSwipeRight() to the action listener interface. Tracks horizontal gesture travel in onModifiedTouchEvent(), aborts default click handling during swipe, fires haptic feedback.
BackspaceHandler.kt	Introduced deletedChunksStack (java.util.Stack). Added performSwipeDelete() (push + delete) and performSwipeRestore() (pop + insert). Stack clears on cursor movement or direct key input.
GeneralKeyboardIME.kt	Declares SwipeTutorialState enum (NOT_ACTIVE, SWIPE_LEFT_STEP, SWIPE_RIGHT_STEP, COMPLETED). Routes gesture events, drives tutorial step progression, auto-inserts "Scribe " at tutorial start.

Layout & assets

File	Changes
input_method_view.xml	Set android:clickable="false" and android:focusable="false" on the tutorial overlay for touch pass-through. Redesigned to show one instruction, one illustration, and one action button at a time.
ic_swipe_left.xml	Updated — three-key outline with indigo/blue gesture trail.
ic_swipe_right.xml	Updated — three-key outline with emerald green gesture trail.
ic_swipe_success.xml	New — three-key outline with emerald green checkmark.

Tests

File	Changes
BackspaceHandlerTest.kt	JUnit/Robolectric tests covering stack push/pop behaviour, deletion offsets, character vs. word-by-word modes, and automatic stack clearing.

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Testing

Automated

./gradlew testKeyboardsDebugUnitTest

Manual checklist

Start from a fresh install or clear the swipe_tutorial_shown preference.

• Tutorial overlay appears on first launch with dark-slate translucent style
• "Scribe " is pre-typed in the active text field
• Keyboard keys respond to touch while the overlay is visible
• Swiping left deletes "Scribe " word-by-word with haptic feedback, then advances to step 2
• Swiping right restores "Scribe " with haptic feedback, then shows the success screen
• Tapping Got it! dismisses the overlay; normal typing and swipes work correctly
• Tapping Skip at any step dismisses the overlay and saves the preference
• Tutorial does not reappear on subsequent launches
• Swipe delete and restore work correctly outside the tutorial in normal use

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Screenshots

Step 1 — Swipe left to delete	Step 2 — Swipe right to restore	Completion screen

[github-actions]

Thank you for the pull request! 💙🩵

The Scribe-Android team will do our best to address your contribution as soon as we can. The following are some important points:

• Those interested in developing their skills and expanding their role in the community should read the mentorship and growth section of the contribution guide
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[github-actions]

Maintainer Checklist

The following is a checklist for maintainers to make sure this process goes as well as possible. Feel free to address the points below yourself in further commits if you realize that actions are needed :)

• Tests for changes have been written and the unit test, linting and formatting workflows within the PR checks do not indicate new errors in the files changed

  • Tests may need to be reran as they're at times not deterministic

• The CHANGELOG has been updated with a description of the changes for the upcoming release and the corresponding issue (if necessary)