accessibility: improve screen reader support for audio scrubber

[UlisesMilani]

First time contributor checklist:

• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

This PR addresses the excessive screen reader verbosity issue when playing audio messages in Signal Desktop.

Fixes #7745

Issues Addressed & Steps to Replicate

• The Problem: When playing a voice message, the screen reader continuously reads the progress bar updates (elapsed time or percentage) out loud. This overlaps and completely drowns out the actual voice message, making it impossible to listen to it.
• Steps to Replicate:
  1. Turn on NVDA.
  2. Open a chat and play any voice message.
  3. Notice that the screen reader repeatedly announces the progress updates as the audio plays.
• The Fix: Added an isPlaying prop to track active playback. The WaveformScrubber now only updates aria-valuenow and aria-valuetext when the audio is paused/stopped or when the user actively interacts with the scrub controls.

Test approach

• Manual testing: Played and scrubbed voice notes and verified that progress updates are silent during playback, but correctly announced when pausing or manually navigating/scrubbing using the keyboard.
• Operating system: Windows 11 with NVDA (NonVisual Desktop Access).